1. Field of the Invention
The invention relates to electronic devices and communication, and more particularly to error correction coding, transmission, storage, and methods.
2. Background
Digital subscriber line (DSL) technologies provide potentially large bandwidth (e.g., up to 20 Mbps for subscribers close to the central office) for digital communication over existing telephone subscriber lines (the copper plant). Telephone subscriber lines can provide this bandwidth despite their original design for only voice band (0-4 kHz) analog communication. In particular, ADSL (asymmetric DSL) adapts to the characteristics of the subscriber line by using a discrete multitone (DMT) line code with the number of bits per tone (sub-carrier) adjusted to channel conditions as determined during training and initialization of the modems at each end of the subscriber line. Additionally, ADSL employs forward error correction coding based on Reed-Solomon codes using GF(256); the use of GF(256) permits the Galois field elements to be represented by bytes, and codewords may have up to 255 bytes. The bits of a codeword are allocated among the sub-carriers for modulation to form an ADSL symbol for transmission.
FIG. 1 shows an ADSL reference model system and FIG. 2 shows a reference transmitter model for transport of ATM data. The functional blocks may be physically performed by specialized circuitry or programmed into a digital signal processor or other processor or by a combination of such elements.
Channel analysis during training and initialization by the central office and remote site modems determines the received signal power and noise power for each upstream and downstream sub-carrier. The variables of interest are the number of bits and proportion of total power allocated to each sub-carrier. Maximize the total bit rate by choosing the values of these variables to approximately equalize the bit error rates among the sub-carriers. Then the probability of a bit error in symbol-by-symbol detection in each sub-carrier depends as usual upon the error function of the average squared distance between the constellation points and the received data points. Interleaving of the bytes prior to allocation of bits to sub-carriers will spread out burst errors.
Generally, DSL modems encounter an extremely wide range of operating conditions. It is important to determine the optimum number of Reed-Solomon parity bytes to use in order to achieve the highest possible data rate. It is also important to be able to accurately predict the performance of a given Reed-Solomon configuration in order to assess its suitability to a given set of channel conditions.
Existing approaches first assume that all Reed-Solomon configurations give similar performance, next, empirically determine the worst observed performance of those configurations that are likely to be used, and then assume this worst observed performance for all configurations.
While this will work reasonably well over a narrow range of channel conditions it does not work well over the full range of channel conditions.
The present invention provides a communication method including determination of codeword configuration (number of parity bytes and total number of bytes) for a given/detected mean squared error and a target error-corrected error rate upper bound.
This has the advantages of providing the maximum possible data rate and predictable modem performance for a given bit error rate.